The present invention relates to an electrical field assisted ion-exchange method of making buried waveguides.
Passive optical waveguide paths have previously been formed in the surface of a glass substrate by ion-exchange processes. Such waveguide paths are useful for integrated optical applications due to their compatibility with optical fibers and their low cost of fabrication. A first surface of a glass substrate is initially masked by depositing a layer of masking material on a surface of the substrate, and photolithographically etching the layer of masking material, leaving openings where the waveguide path is to be formed. The masked surface is contacted by a first molten salt bath. In most cases, sodium ions in the substrate glass are exchanged for a dopant cation such as Cs, Ag, Ru or Tl. An electrical field is sometimes applied during this first ion-exchange process. The waveguide path can be buried under the substrate surface by removing the mask, applying an electrode to the second surface of the substrate and contacting the active side with a second molten salt bath which contains ions that contribute less to the substrate refractive index, eg. Na and K ions. While in contact with the second salt bath, an electrical field is applied between the bath and the second surface electrode.
As a result of this double ion-exchange process, there is formed beneath the first substrate surface a signal carrying region that has a higher refractive index than the region surrounding it. It is advantageous to have fairly sharp boundaries between the higher refractive index region and its neighboring regions. Moreover, the higher refractive index region should be buried deeply enough beneath the substrate surface to prevent scattering from surface irregularities and defects. A depth greater than 15 .mu.m is conventionally employed to prevent the electromagnetic field from reaching the surface of the glass. The refractive index profile depends on such parameters as the composition of the substrate glass, the nature of the incoming dopant ion, its concentration in the source, salt bath temperature, diffusion time, and the magnitude of the externally applied field.
The adverse effects of thermal diffusion can be reduced by burying a waveguide at a relatively low temperature. However, the processing time required to achieve a given depth correspondingly increases. An electrical field has been used to increase the rate of ion movement at low temperatures. U.S. Pat. No. 4,913,717 states that waveguides having sharp, well defined boundaries can be formed by performing the first ion-exchange at low temperature and under the influence of an electrical field of up to a few hundred volts per millimeter and burying the waveguide at a low temperature and again applying the voltage.
Although further increase in electrical field strength can decrease processing time, it can result in a number of problems: (a) it can cause arcing, (b) it can cause breakage of the glass substrate by having short circuits between the two sides of the glass substrate, these short-circuits being due to ion migration up the edges of the substrate, and (c) it can cause process instability due to Joule effect heating of the substrate.
Joule effect heating due to the application of high electrical fields can have the following effect on the process. As a high voltage is applied to the wafer, Joule effect heating of the wafer increases current flow due to the higher mobility of the ions. More power is then dissipated in the wafer, and the temperature of the wafer continues to increase. This can result in (a) a loss of control of the process due to overheating of the substrate, such overheating sometimes leading to substrate breakage, (b) a non-uniformity of waveguide characteristics (index of refraction, waveguide dimension, and depth of burial) due to the temperature gradients created, and (c) a warping of the substrate, whereby attachment of optical fibers to the waveguide paths becomes more difficult.